Gas separations with redox-active metal-organic frameworks

ABSTRACT

Fe 2 (dobdc) has a metal-organic framework with a high density of coordinatively-unsaturated Fe II  centers lining the pore surface. It can be effectively used to separate O 2  from N 2  and in a number of additional separation applications based on selective, reversible electron transfer reactions. In addition to being an effective O 2  separation material, it can be used for many other processes, including paraffin/olefin separation, nitric oxide/nitrous oxide separation, acetylene storage, and as an oxidation catalyst.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 61/527,331, filed Aug. 25, 2011, the contents of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to Fe₂(dobdc), a novel material that has ametal-organic framework with a high density ofcoordinatively-unsaturated Fe^(II) centers lining the pore surface. Thismaterial can be used for the separation of O₂ from N₂ and in a number ofadditional separation applications based on selective, reversibleelectron transfer reactions. In addition, it can be used for many otherprocesses, including paraffin/olefin separations, nitric oxide/nitrousoxide separation, carbon monoxide removal, acetylene storage, and as anoxidation catalyst.

BACKGROUND OF THE INVENTION

With over 100 million tons produced annually, O₂ is one of the mostwidely used commodity chemicals in the world.¹ Its potential utility inprocesses associated with the reduction of carbon dioxide emissions fromfossil fuel-burning power plants, however, means that the demand forpure O₂ could grow enormously. For example, when implementingpre-combustion CO₂ capture, pure O₂ is used for the gasification ofcoal, which produces the feedstock for the water-gas shift reaction usedto produce CO₂ and H₂.² In addition, oxyfuel combustion has receivedconsiderable attention for its potential utility as an alternative topost-combustion CO₂ capture. Here, pure O₂ is diluted to 0.21 bar withCO₂ and fed into a power plant for fuel combustion. Since N₂ is absentfrom the resulting flue gas, the requirement for post-combustionseparation of CO₂ from N₂ is eliminated.³

The separation of O₂ from air is currently carried out on a large scaleusing an energy-intensive cryogenic distillation process.⁴ Zeolites arealso used for O_(2/)N₂ separation,⁵ both industrially and in portablemedical devices. However, this process is inherently inefficient as thematerials used adsorb N₂ over O₂ with poor selectivity. By employingmaterials that selectively adsorb O₂ and can operate near ambienttemperatures, lower energy and capital costs could be realized.Metal-organic frameworks (“MOFs”), which have already receivedconsiderable attention for applications in gas storage⁶ and separation,⁷represent a promising new class of potential O₂ separation materials.

The energy cost associated with the separation of hydrocarbons, ascurrently carried out at enormous scale via cryogenic distillation,could potentially be lowered through development of selective solidadsorbents that operate at higher temperatures and lower pressures. As aconsequence of the similar sizes and volatilities of the hydrocarbons,separations, for example, of olefin/paraffin mixtures, such asethylene/ethane and propylene/propane, must currently be performed atlow temperatures and high pressures, and are among the mostenergy-intensive separations carried out at large scale in the chemicalindustry. Because these hydrocarbon gaseous mixtures are produced bycracking long-chain hydrocarbons at elevated temperatures, a substantialenergy penalty arises from cooling the gases to the low temperaturesrequired for distillation. Thus, tremendous energy savings could berealized if materials enabling the efficient separation of hydrocarbonsat higher temperatures, than currently used in distillation, andatmospheric pressure were achieved.

Current competing approaches for separating hydrocarbons includemembrane designs, organic solvent-based sorbents, as well as poroussolid adsorbents featuring selective chemical interactions with thecarbon-carbon double bond in olefins. In this latter category, MOFs,which offer high surface areas, adjustable pore dimensions, and chemicaltenability, have received considerable attention as adsorbents in gasstorage and separation applications, with particular emphasis on thedense storage of methane and hydrogen, and on the efficient removal ofcarbon dioxide from flue gas and natural gas deposits. More recently,MOFs represent a promising new class of potential hydrocarbon separationmaterials.

In addition to the separation of binary olefin/paraffin mixtures, thereis tremendous current interest in separating ethane, ethylene, andacetylene from methane for the purification of natural gas. Indeed, anumber of porous materials are able to selectively separate methane frommixtures including C₂ hydrocarbons (ethane, ethylene, and acetylene).These materials, however, are unable to simultaneously purify theethane, ethylene, and acetylene being removed from the gas stream. Aseparation process that utilizes the same adsorptive material for theseparation and purification of all four components of a C₁/C₂ mixturecould potentially lead to substantial efficiency and energy savings overcurrent processes.

Ethylene produced in a naphtha cracker contains an impurity ofapproximately 1% acetylene. However, there are strict limitations to theamount of acetylene that can be tolerated in the feed to an ethylenepolymerization reactor. The current technology for this purpose usesabsorption with liquid DMF, but the use of solid adsorbents couldpotentially provide an energy-efficient alternative.

In addition, early efforts in developing metal-organic frameworkcatalysts have largely focused on new synthetic methods forincorporating catalytic functionalities onto the pore surface, as wellas proof-of-concept studies, such as the heterogenization of well-knownhomogeneous catalysts or simple acid/base activation of substrates.While these examples demonstrate the viability of metal-organicframeworks as heterogeneous catalysts, they provide little improvementover existing systems and do not take full advantage of propertiesunique to metal-organic frameworks, including the ability to designspecific and spatially separated active sites. In particular, frameworkincorporation of reactive transition metal intermediates, such asmetal-ligand multiple bonds or low-coordinate metal centers, is apromising area that has yet to be explored. In principle, redoxcatalysis involving the formation of metal species in unusualcoordination environments, geometries, and/or oxidation states that areentirely unfeasible in homogeneous systems could proceed easily in thecontext of metal-organic frameworks wherein each metal center is heldfixed and isolated.

SUMMARY OF THE INVENTION

Metal-organic frameworks have received considerable attention for avariety of gas separation applications. However, the use of Fe₂(dobdc),a metal-organic framework featuring redox-active metal centers for gasseparations based on selective, reversible (partial) electron transferreactions represents a novel advance in the field. This material may beused for numerous separation and storage applications including, but notlimited to, paraffin/olefin separations, oxygen/nitrogen separation,carbon monoxide removal, acetylene storage, and nitric oxide/nitrousoxide separations. This material displays incredible separationproperties at temperatures that are much more favorable to thosecurrently used in industrial applications.

One embodiment is a material includingFe₂(2,5-dioxido-1,4-benzenedicarboxylate). A method of making Fe₂(dobdc)(dobdc=2,5-dioxido-1,4-benzenedicarboxylate) may include reacting FeCl₂with H₄dobdc (dobdc=2,5-dioxido-1,4-benzenedicarboxylate) in a reactionmixture to produce Fe₂(dobdc). The reaction mixture may also includedimethylformamide (DMF) and methanol.

Fe₂(dobdc) may use to separate a variety of mixtures. A embodiment of amethod of separating a mixture stream including O₂ and N₂ may includecontacting a mixture stream comprising O₂ and N₂ with a materialcomprising Fe₂(dobdc) to obtain a stream richer in O₂ as compared to themixture stream, and obtain a stream richer in N₂ as compared to themixture stream.

An embodiment of a method of separating a mixture including a firstchemical and a second chemical may include contacting a mixture streamincluding the first chemical and the second chemical with a materialcomprising Fe₂(dobdc), obtaining a stream richer in the first chemicalas compared to the mixture stream, and obtaining a stream richer in thesecond chemical as compared to the mixture stream.

For example, the first chemical may be a paraffin and the secondchemical may be an olefin. The first chemical may be ethane and thesecond chemical may be ethene. The first chemical may be propane and thesecond chemical may be propene. The first chemical may be nitric oxideand the second chemical may be nitrous oxide.

An embodiment of a method of storing acetylene may include contactingacetylene with Fe₂(dobdc).

A method of oxidizing a material may include contacting the materialwith Fe₂(dobdc).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of a portion of the crystalstructure of desolvated Fe₂(dobdc) as viewed approximately along the[001] direction.

FIG. 2 is a diffuse reflectance UV-visible-NIR spectra of methanolsolvated, desolvated Fe₂(dobdc), and H₄dobdc.

FIG. 3 is a graph showing excess O₂ and N₂ adsorption isothermscollected for Fe₂(dobdc) at various temperatures.

FIG. 4 is graph showing the uptake and release of O₂ in Fe₂(dobdc) over13 cycles at 211 K.

FIG. 5 is a graph that shows calculated N₂ and O₂ breakthrough curvesduring adsorption of simulated air by Fe₂(dobdc).

FIG. 6 is a Mössbauer spectra measured between 94 and 252 K forFe₂(dobdc) in the presence of O₂.

FIG. 7 is an infrared spectra obtained for Fe₂(dobdc) with and withoutO₂ and at various temperatures.

FIG. 8 is a series of graphical representations of a first coordinationspheres for the iron centers within Fe₂(dobdc) and its O₂ and N₂ dosedvariants, dosed at various temperatures.

FIG. 9 is a graph showing the separation of a mixture of ethane andethane.

FIG. 10 is a graph showing the separation of a mixture of propane andpropene.

FIG. 11 is a graph comparing the powder X-ray diffraction patterns ofas-synthesized Fe₂(dobdc), Zn₂(dobdc), and Mg₂(dobdc).

FIG. 12 is a Rietveld refinement of the experimental neutron diffractionpattern of desolvated Fe₂(dobdc).

FIG. 13 is a graph showing N₂ adsorption in Fe₂(dobdc) at 77 K.

FIG. 14 is a TGA of Fe₂(dobdc) under N₂.

FIG. 15 a graph showing dual-site Langmuir-Freundlich fits for Oxygenand Nitrogen adsorption in Fe₂(dobdc).

FIG. 16 is a graph showing isosteric heats of adsorption of O₂ and N₂.

FIG. 17 is a graph of O₂/N₂ selectivity as a function of pressure atvarious temperatures.

FIG. 18 is a schematic drawing of VSA apparatus.

FIG. 19 is a series of graphs showing a calculated N₂ adsorptionbreakthrough curves and calculated O2 desorption breakthrough curvesduring adsorption/desorption of simulated air by Fe₂(dobdc) at varioustemperatures.

FIG. 20 is graph showing temperature dependence of quadrupole splittingfor Fe₂(dobdc), Fe₂(O₂)₂(dobdc), and Fe₂(O₂)(dobdc).

FIG. 21 is a graph showing temperature dependence of the logarithm ofthe Mössbauer spectral absorption area of Fe₂(dobdc).

FIG. 22 is a graph showing temperature dependence of the isomer shiftsFe₂(dobdc), Fe₂(O₂)₂(dobdc), and Fe₂(O₂)(dobdc).

FIG. 23 is a graph showing relative absorption areas of Fe₂(dobdc),Fe₂(O₂)₂(dobdc), and Fe₂(O₂)(dobdc) as a function of temperature.

FIG. 24 shows graphs of the effects of progressive dosage of O₂ at lowtemperature and of outgassing at low temperature on the IR spectra ofactivated Fe₂(dobdc).

FIG. 25 is an ATR spectra of activated Fe₂(dobdc) and a partiallyoxidized sample.

FIG. 26 is series of Raman spectra of activated Fe₂(dobdc), a partiallyoxidized sample, and an activated sample subject to 40 mbar of oxygen.

FIG. 27 is Rietveld refinement of the experimental neutron diffractionpattern of Fe₂(dobdc) exposed to O₂ at 100K.

FIG. 28 is Rietveld refinement of the experimental neutron diffractionpattern of Fe₂(dobdc) exposed to O₂ at 298K.

FIG. 29 shows, at the top, a graphical representation of a structure ofFe₂(O₂)₂(dobdc)-20₂ as viewed down the (001) direction. At the bottomare an H₄(dobdc) ligand and the first coordination spheres for the ironcenters in the solid state structures obtained upon dosing Fe₂(dobdc)with acetylene, ethylene, ethane, propylene, and propane.

FIG. 30 is a series of graphs, the top row of which are gas adsorptionisotherms for methane, ethane, ethylene, and acetylene (a) and forpropane and propylene (b) in Fe₂(dobdc) at 318 K, and the bottom row ofwhich are experimental breakthrough curves for the adsorption ofequimolar ethane/ethylene (c) and propane/propylene (d) mixtures flowingthrough Fe₂(dobdc).

FIG. 31 is a series of graphs showing calculations of the adsorptionselectivity for ethane/ethylene (upper left), propane/propylene (upperright), acetylene/ethylene (lower left) and acetylene/methane,ethylene/methane, ethane/methane (lower right) in Fe₂(dobdc) at 318 K.

FIG. 32 is a series of curves showing mol % (left) and concentration(right) of propane and propylene during adsorption (upper) anddesorption (lower) of a simulated breakthrough experiment.

FIG. 33 is a series of curves showing mol % (left) and concentration(right) of ethane and ethylene during adsorption (upper) and desorption(lower) of a simulated breakthrough experiment.

FIG. 34 is a series of graphs showing production capacities of 99% purepropane (left) and 99.5% pure propylene (right) as a function of thetotal pressure at the inlet to the adsorber for Fe₂(dobdc), Mg₂(dobdc),NaX zeolite, Cu₃(btc)₂, Cr₃(btc)₂, ITQ-12, and Fe-MIL-100.

FIG. 35 is a series of graphs showing production capacities of 99% pureethane (left) and 99.5% pure ethylene (right) as a function of the totalpressure at the inlet to the adsorber for Fe₂(dobdc), Mg₂(dobdc), andNaX zeolite.

FIG. 36 shows, at left, a graph of calculated methane, ethane, ethylene,and acetylene breakthrough curves for various gases in Fe₂(dobdc), and,at right, a schematic representation of the separation of a mixture ofmethane, ethane, ethylene, and acetylene using Fe₂(dobdc) in anadsorption process.

FIG. 37 shows transient breakthrough of acetylene/ethylene mixture in anadsorber bed packed with Fe₂(dobdc).

FIG. 38 is a graphical representation of a portion of the structure ofFe₂(dobdc) showing carbon monoxide (CO) coordinated to the open Fe²⁺site.

FIG. 39 is an infrared spectrum of Fe₂(dobdc) under successivelyincreasing doses of CO.

FIG. 40 is graph of low-pressure adsorption of CO, CH₄, N₂, and H₂ inFe₂(dobdc) at 298 K.

FIG. 41 shows the excess adsorption isotherms of CO, CH₄, N₂, and H₂collected for Fe₂(dobdc) at 318 K, and shows (inset) the isosteric heatof CO adsorption in Fe₂(dobdc) as a function of loading.

FIG. 42 show the infrared spectrum of CO adsorbed on Fe₂(dobdc) at roomtemperature.

FIG. 43 is a graphical representation of crystal structures ofFe₂(dobdc) with adsorbed N₂, CO, or D₂.

FIG. 44 is an ¹H NMR of products of the reaction of Fe₂(dobdc) withexcess 1,4-cyclohexadiene in CD₃CN.

FIG. 45 is a table showing dual-site Langmuir-Freundlich parameters forO2 isotherms in Fe2(dobdc) at different temperatures.

FIG. 46 is a table shownig dual-site Langmuir-Freundlich parameters forN2 isotherms in Fe2(dobdc) at different temperatures.

FIG. 47 is a table showing Mössbauer Spectral Parameters for Fe2(dobdc)Obtained Before and After Oxygenation.

FIG. 48 is a table showing unit cell lengths and volumes.

FIG. 49 is a diagram showing the oxidation of propylene to acetone usingO₂ as the oxidant.

FIG. 50 shows representative oxidation reactions.

DETAILED DESCRIPTION OF THE INVENTION

Described is a method of separating a target component from a chemicalmixture using Fe₂(dobdc). FIG. 1 is a portion of the crystal structureof desolvated Fe₂(dobdc) as viewed approximately along the [001]direction (H atoms have been omitted for clarity.) This material has ametal-organic framework with a high density ofcoordinatively-unsaturated Fe^(II) centers lining the pore surface. Thismaterial can be used for the separation of O₂ from N₂ and in a number ofadditional separation applications based on selective, reversibleelectron transfer reactions. In addition to being used as an O₂separation material, it can be used for many other processes, includingparaffin/olefin separations, nitric oxide/nitrous oxide separation,carbon monoxide removal, acetylene storage, and as an oxidationcatalyst.

The high surface areas and open metal coordination sites possible withinmetal-organic frameworks make them particularly attractive for thedevelopment of an adsorption-based process for the separation of O₂ fromair. While coordinatively-unsaturated metal centers have been generatedin such materials via chelation by post-synthetically modified bridgingligands,⁸ or via insertion into open ligand sites,⁹ they are most oftencreated by evacuation of frameworks that have metal-bound solventmolecules. This strategy has been employed to expose M²⁺ cation sites insome of the most widely-studied frameworks, such as M₂(dobdc) (M=Mg, Mn,Co, Ni, Zn; dobdc⁴⁻=2,5-dioxido-1,4-benzenedicarboxylate)¹⁰ and M₃(BTC)₂(M=Cu, Cr, Mo; BTC³⁻=1,3,5-benzenetricarboxylate).¹¹

To achieve a high selectivity for the coordination of O₂ over N₂, onecan take advantage of the greater electron affinity of the formermolecule. Indeed, coordinatively-unsaturated Cr^(II) centers inCr₃(BTC)₂ give rise to an exceptionally strong preference for adsorbingO₂ relative to N₂ via charge transfer. Although the interaction with O₂proved too strong to achieve full reversibility with this material, theresult demonstrates the potential power of frameworks with redox-activemetal centers for the separation of O₂ and N₂. In view of its widespreaddeployment as an O₂ carrier in biology,¹² Fe^(II) was chosen.

The air-free reaction between FeCl₂ and H₄dobdc(dobdc⁴⁻=2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of DMF andmethanol affords Fe₂(dobdc).4DMF, a metal-organic framework adopting theMOF-74 (or CPO-27) structure type. The desolvated form of this materialdisplays a BET surface area of 1350-1360 m²/g and features a hexagonalarray of one-dimensional 11 Å wide channels lined withcoordinatively-unsaturated Fe^(II) centers. With a compact tetra-anionicbridging ligand, the structure features a unprecedented high surfacedensity of 2.9 Fe^(II) coordination sites available per 100 11 Å² on itssurface, with spacings of just 6.84(1) and 8.98(2) Å between iron atomsalong and around a channel, respectively. Thus, it appears to provide anear optimal platform for the high-capacity adsorption of small olefins,such as ethylene and propylene. Furthermore, the Mg²⁺ or Co²⁺ analoguesof this structure type have recently been shown to disply selectiveadsorption for olefins over paraffins. The higher the surface area andsofter metal character of Fe₂(dobdc) as compared to the recentlyreported materials should lend both higher selectivity and capacity tothe iron(II) framework.

Gas adsorption isotherms at 298 K indicate that Fe₂(dobdc) binds O₂preferentially over N₂, with an irreversible capacity of 9.3 wt %,corresponding to the adsorption of one O₂ molecule per two iron centers.Remarkably, at 211 K, O₂ uptake is fully reversible and the capacityincreases to 18.2 wt %, corresponding to the adsorption of one O₂molecule per iron center. Mössbauer and infrared spectra are consistentwith partial charge transfer from iron(II) to O₂ at low temperature andcomplete charge transfer to form iron(III) and O₂ ²⁻ at roomtemperature. The results of Rietveld analyses of powder neutrondiffraction data (4 K) confirm this interpretation, revealing O₂ boundto iron in a symmetric side-on mode with d_(O-O)=1.25(1) Å at lowtemperature and in a slipped side-on mode with d_(I-O)=1.6(1) Å whenoxidized at room temperature. Application of ideal adsorbed solutiontheory in simulating breakthrough curves show Fe₂(dobdc) to be apromising material for the separation of O₂ from air at temperatureswell above those currently employed in industrial settings.

Herein, we report the synthesis and O₂ binding properties of Fe₂(dobdc),a metal-organic framework with a high density ofcoordinatively-unsaturated Fe^(II) centers lining the pore surface.

This invention will be better understood with reference to the followingexperimental examples, which are intended to illustrate specificembodiments within the overall scope of the invention.

Experimental Section

General. Unless otherwise noted, all procedures were performed under anN₂ atmosphere using standard glovebox or Schlenk techniques. Anhydrous,air-free N,N-dimethylformamide (DMF) and methanol were purchased fromcommercial vendors and further deoxygenated by purging with N₂ for atleast 1 h prior to being transferred to an inert atmosphere glovebox.All other reagents were obtained from commercial vendors at reagentgrade purity or higher and used without further purification.

Synthesis of Fe₂(dobdc). Anhydrous ferrous chloride (1.1 g, 9.0 mmol),1,4-dihydroxyterephthalic acid (0.71 g, 3.6 mmol), DMF (300 mL) andmethanol (36 mL) were added to a 500-mL Schlenk flask. The reactionmixture was heated at 393 K and stirred for 18 h to afford a red-orangeprecipitate. The solid was collected by filtration and washed with 100mL of DMF to yield 2.0 g (91%) of Fe₂(dobdc)·4DMF. Anal. Calcd. forC₂₀H₃₀Fe₂N₄O₁₀: C, 40.16; H, 5.06; N, 9.37. Found: C, 40.26; H, 5.08; N,9.24. A sample of this compound (1.9 g, 3.3 mmol) was soaked in 100 mLof DMF at 393 K for 24 h after which the solvent was decanted, and thesolid was then soaked in 100 mL of methanol at 343 K for 24 h. Themethanol exchange was repeated three times, and the solid was collectedby filtration to yield 1.25 g (87%) of Fe₂(dobdc)·4MeOH as ayellow-ochre powder. Anal. Calcd. for Fe₂Cl₂H₁₈O₁₀: C, 33.21; H, 4.18.Found: C, 33.42; H, 4.09. A sample of this compound was fully desolvatedby heating under dynamic vacuum (<10 μbar) at 433 K for 24 h to yieldFe₂(dobdc) as a light green powder. Anal. Calcd. for Fe₂C₈H₂O₆: C,31.42; H, 0.66. Found: C, 31.55; H, 0.50.

Low-Pressure Gas Adsorption Measurements. For all gas adsorptionmeasurements 200-225 mg of Fe₂(dobdc)·4MeOH was transferred to apre-weighed glass sample tube under an atmosphere of nitrogen and cappedwith a Transeal. Samples were then transferred to Micromeritics ASAP2020 gas adsorption analyzer and heated at a rate of 0.1 K/min from roomtemperature to a final temperature of 433 K. Samples were consideredactivated when the outgas rate at 433 K was less than 2 Oar/min.Evacuated tubes containing degassed samples were then transferred to abalance and weighed to determine the mass of sample, typically 150-175mg. The tube was transferred to the analysis port of the instrumentwhere the outgas rate was again determined to be less than 2 Oar/min at433 K. Nitrogen gas adsorption isotherms at 77 K were measured in liquidnitrogen, while O₂ measurements between 200 and 273 K were measuredusing liquid nitrogen/solvent slurry baths. All measurements above 273 Kwere performed using a recirculating dewar connected to an isothermalbath.

Transmission Infrared and Diffuse Reflectance UV-vis-NIR Spectroscopy.Prior to O₂ dosing, Fe₂(dobdc)·4MeOH samples were activated underdynamic vacuum (residual pressure<0.1 μbar) at 433 K for 18 h. Infraredspectra were collected on thin deposits of sample supported on a siliconwafer in an airtight quartz cell that allows for collection of spectraunder controlled atmospheres. The film was prepared from a suspension ofFe₂(dobdc) in methanol. Transmission FTIR spectra were collected at2-cm⁻¹ resolution on a Bruker IFS 66 FTIR spectrometer equipped with aDTGS detector. Diffuse Reflectance UV-vis-NIR spectra were recorded on aCary 5000 spectrophotometer equipped with reflectance sphere. Spectra ofthe desolvated framework were recorded on a thick self-supported waferof the sample. Attenuated total reflection (ATR) spectra were recordedon a Bruker single reflection ALPHA-Platinum ATR spectrometer with adiamond crystal accessory.

Neutron Diffraction Data Collection and Refinement. Neutron powderdiffraction (NPD) experiments were carried out on 0.9698 g and 0.6200 gof Fe₂(dobdc) and Fe₂(O₂)(dobdc) respectively using the high-resolutionneutron diffractometer, BT1, at the National Institute of Standards andTechnology Center for Neutron Research (NIST). Both samples were placedin a He purged glove box, loaded into a vanadium can equipped with a gasloading valve, and sealed using an indium O-ring. Neutron diffractiondata were collected using a Ge(311) monochromator with an in-pile 60′collimator corresponding to a wavelength of 2.0782 Å. The samples wereloaded into a top-loading closed cycle refrigerator and then data werecollected at 4 K. After data collection of the bare material, O₂ loadingwas then carried out. The sample was warmed to 125 K and then exposed toa predetermined amount of gas (2.0 O₂ per Fe²⁺). Upon reaching anequilibrium pressure at the loading temperature, the sample was thenslowly cooled to ensure complete adsorption of the O₂. Data was thencollected at 4 K.

NPD measurements of N₂-loaded Fe₂(dobdc) were performed on the Echidnainstrument¹⁴ located at the Opal research reactor and operated by theBragg Institute within the Australian Nuclear Science and TechnologyOrganisation (ANSTO). A desolvated sample weighing 1.079 g wastransferred to a vanadium cell in an Ar-filled glovebox. The cell wasequipped with heaters for the gas line and valve to allow condensablegases to be loaded in the sample when mounted in the closed cyclerefrigerator. The high-resolution diffractometer was configured with aGe(331) monochromator using a take-off angle of 140° with no collimationat the monochromator and fixed tertiary 5′ collimation, resulting in awavelength of 2.4406 Å. Diffraction data were collected at ˜9 K for theevacuated framework and with sequential loadings of 0.5, 1.0 and 2.0N₂:Fe, where the cryostat and sample were heated above 80 K tofacilitate adsorption of the 99.999% pure N₂ gas.

All NPD data were analyzed using the Rietveld method as implemented inEXPGUI/GSAS. The activated Fe₂(dobdc) model was refined with allstructural and peak profile parameters free to vary, resulting in astructure very similar to that determined using single crystal X-raydiffraction. Fourier difference methods were then employed to locate theadsorbed molecules in the data collected from the samples subsequentlyloaded with O₂ or N₂. The atoms in the adsorbed molecules were modeledindividually. For the N₂ adsorbed sample the two N atoms wereconstrained to maintain the fractional occupancy and isotropicdisplacement parameter within each diatomic molecule. For analysis ofFe₂(dobdc) loaded with 2.0 O₂/Fe, only fractional occupancies wereconstrained to maintain the same values, while all other parameters wereallowed to vary. Further, for data collection of the irreversiblyoxidized sample, Fe₂(O₂)(dobdc), the modeled O atoms were constrained tomaintain the same fractional occupancies and isotropic displacementparameters. Once a stable structural model was obtained the isotropicdisplacement parameters of the adsorbed O₂ molecule were allowed to varyindependently of one another and then the displacement parameter forO(1b) were allowed to refine anisotropically.

Mössbauer Spectroscopy. The Mössbauer spectra of Fe₂(dobdc),Fe₂(O₂)₂(dobdc), and Fe₂(O₂)(dobdc) were measured at varioustemperatures between 94 and 298 K with a constant accelerationspectrometer which utilized a rhodium matrix cobalt-57 source, and wascalibrated at 295 K with α-iron foil. The absorber contained 45(1)mg/cm² of powder mixed with boron nitride. The Fe₂(dobdc) absorber wasprepared in an N₂-filled glovebox, cooled to 77 K with liquid nitrogen,and inserted into a pre-cooled cryostat under dry helium. The sample ofFe₂(O₂)₂(dobdc) was prepared in situ by dosing the evacuated cryostat to300 mbar O₂ at 94 K and allowing 3 h for equilibration. The sample ofFe₂(O₂)(dobdc) was prepared in situ by warming the oxidized sample above250 K in the cryostat. The spectra of Fe₂(dobdc) were measured at 298,94, and 45 K in the absence of O₂, after which the sample was warmed to94 K and dosed with O₂. Additional spectra were measured between 94 and298 K and then subsequently measured again at 94 and 298 K. All spectrawere fit with symmetric Lorenzian quadrupole doublets; the resultingspectral parameters, listed in the order of measurement, are given inFIG. 47. FIG. 47 shows Mössbauer Spectral Parameters for Fe2(dobdc)Obtained before and after oxygenation. The temperature dependence of theobserved isomer shifts and relative absorption areas are plotted inFIGS. 22 and 23, respectively; further spectral details are provided inthe Supporting Information. FIG. 22 is a temperature dependence of theisomer shifts Fe₂(dobdc), Fe₂(O₂)₂(dobdc), and Fe₂(O₂)(dobdc); and FIG.23 is a relative absorption areas of Fe₂(dobdc), Fe₂(O₂)₂(dobdc), andFe₂(O₂)(dobdc) as a function of temperature; The relative statisticalerrors associated with the isomer shifts, quadrupole splittings, linewidths, percent areas, and absolute areas between 94 and 298 K are alsogiven in FIG. 47.

Other Physical Measurements. Thermogravimetric analysis was carried outat a ramp rate of 1° C./min in a nitrogen flow with a TA instruments TGA5000. Powder X-ray diffraction patterns were collected on air-freesamples sealed in quartz capillaries on a Bruker Advance D8 powder X-raydiffractometer equipped with a capillary stage.

Results and Discussion

Synthesis of Fe₂(dobdc). The reaction of anhydrous FeCl₂ with H₄dobdc ina mixture of DMF and methanol affords a solvated form of Fe₂(dobdc) as ared-orange microcrystalline powder. FIG. 11 is a comparison between thepowder X-ray diffraction patterns of as-synthesized Fe₂(dobdc),Zn₂(dobdc), and Mg₂(dobdc). Powder x-ray diffraction data (see FIG. 11)show the compound to adopt the MOF-74 or CPO-27 structure type displayedin FIG. 1, as previously reported for M₂(dobdc) (M=Mg, Mn, Co, Ni,Zn).¹⁰ The compound rapidly changes color to dark brown upon exposure toair, presumably due to at least partial oxidation of the Fe^(II) centersby O₂. Based upon color, it is likely that the brown phase previouslyreported as Fe₂(dobdc) is actually some oxidized form of thecompound.^(10h) Note that, perhaps owing to their air-sensitive nature,only a very few metal-organic frameworks based upon iron(II) have yetbeen isolated.¹⁵

The new framework was completely desolvated by soaking it in methanol toexchange coordinated DMF, followed by heating under dynamic vacuum at433 K for 48 h. The resulting solid was light green in color. FIG. 12 isa Rietveld refinement of the experimental neutron diffraction pattern ofdesolvated Fe₂(dobdc). The calculated pattern is in good agreement withthe experimental data (crosses) as evidenced by the difference pattern.Rietveld analysis of the powder neutron diffraction data collected forFe₂(dobdc) indicate retention of the framework structure with noresidual bound solvent (see FIG. 12). Thus, desolvation converts theFe^(II) centers of the framework from an octahedral coordinationgeometry with one bound solvent molecule to a square pyramidal geometrywith an open coordination site.

FIG. 13 is N₂ adsorption in Fe₂(dobdc) at 77 K. Low-pressure N₂adsorption data obtained for Fe₂(dobdc) at 77 K reveal a type Iadsorption isotherm characteristic of a microporous solid. The dataindicate a BET surface area of 1360 m²/g (1535 m²/g Langmuir) (see FIG.13). This value is significantly higher than the 920 m²/g Langmuirsurface area reported for the material prepared in the presence of airand is in close agreement with the BET surface areas of 1218 m²/g and1341 m²/g reported for Ni₂(dobdc) and Co₂(dobdc), respectively,indicating full evacuation of solvent molecules from the pores of thematerial.¹⁶ FIG. 14 is a TGA of Fe₂(dobdc) under N₂.

UV-vis-NIR Spectroscopy. FIG. 2 is a diffuse reflectance UV-visible-NIRspectra of methanol solvated, desolvated Fe₂(dobdc), and H₄dobdc. FIG. 2shows the electronic absorption spectra for Fe₂(dobdc)·4MeOH,Fe₂(dobdc), and H₄dobdc. The spectrum for the yellow-ochre compoundFe₂(dobdc)·4MeOH exhibits a low energy doublet with peaks at 11600 cm⁻¹and 7600 cm⁻¹. High-spin Fe^(n) centers in an octahedral symmetry areexpected to show a spin-allowed transition, ⁵E_(g)←⁵T_(2g), in the nearinfrared region,¹⁷ and in many compounds this band is split into adoublet due to a lower symmetry ligand field, which lifts the two-foldorbital degeneracy of the ⁵E_(g) term. At higher energy, a broadcomponent centered at 16000 cm⁻¹ and a strong band with a maximum around21000 cm⁻¹ appear in the spectrum. The structure and position of theseabsorptions suggest they arise from mixing of d-d and charge transfer(LMCT and MLCT) transitions.¹⁸ Heating the solvated material at 433 K invacuo results in removal of coordinated methanol with the formation offive-coordinative Fe^(n) centers. The corresponding change in symmetryat the metal site to approximately C_(4ν) strongly affects theelectronic transitions, which is evident from the spectrum of thedesolvated material. In particular, the band at 21000 cm⁻¹ slightlyshifts to lower energy, mixing with the component at 16000 cm⁻ and withthe d-d transition, resulting in a strong absorption extending through13000 cm⁻¹. The very strong absorption maximum at 4400 cm⁻¹ isassociated with a d-d transition, with enhanced intensity owing to lossof an approximate inversion center in the ligand field upon conversionfrom pseudooctahedral to square pyramidal coordination.

O₂ and N₂ Adsorption. Gas adsorption isotherms indicate that Fe₂(dobdc)preferrentially binds O₂ over N₂ at all temperatures measured (201, 211,215, 226, and 298 K). FIG. 3 is a graph showing excess O₂ adsorptionisotherms collected for Fe₂(dobdc) at 211, 226, and 298 K, and N₂adsorption at 298 K. Filled and open circles represent adsorption anddesorption, respectively. As shown in FIG. 3, the O₂ adsorption isothermmeasured at 298 K is extremely steep, climbing to near 9.3 wt % at apressure of just 0.01 bar. As the pressure is increased to 1.0 bar,uptake increases slightly to 10.4 wt %. The steep initial rise in theisotherm is consistent with strong binding of O₂ to some of the Fe^(II)centers, while the subsequent gradual increase in adsorption is likelydue to O₂ physisorbed to the framework surface. Importantly, the amountof strongly bound O₂ corresponds to 0.5 molecules per iron center.Adsorption of N₂ under these conditions is noticeably lower, graduallyrising to just 1.3 wt % at 1.0 bar. The selectivity factor of thismaterial, calculated as the mass of O₂ adsorbed at 0.21 bar divided bythe mass of N₂ adsorbed at 0.79 bar, is 7.5. Although this selectivityfactor is among the highest reported for metal-organic frameworks,¹⁹room temperature O₂ adsorption was found to be irreversible. Attempts toidentify conditions to release coordinated O₂ by heating at temperaturesof up to 473 K under dynamic vacuum ultimately lead to decomposition ofthe framework.

Upon dosing Fe₂(dobdc) with O₂ at lower temperatures, it was noted thatthe black color characteristic of the oxidized framework could bereturned to light green by applying vacuum to the sample, suggestingreversible O₂ adsorption. Additional O₂ adsorption experiments confirmedthis result. As shown in FIG. 3, at 226 K the framework adsorbs 14.1 wt% O₂ at 0.21 bar, or 0.82 O₂ molecules per iron site. Althoughadsorption at this temperature is largely reversible, O₂ uptakedecreases to 11.9 wt % after four adsorption/desorption cycles. Loweringthe adsorption temperature to 211 K results in an increased O₂ uptake of18.2 wt %, corresponding to 1.0 molecules of O₂ per iron center. Theamount of O₂ adsorbed at this temperature was found to decrease onlyslightly to 17.5 wt % after eight adsorption/desorption experiments.However, by cycling at a rapid rate, allowing just 2 min for adsorptionand 25 min for desorption (instead of the 4-5 h typically required forcollecting a full isotherm), resulted in no noticeable loss inadsorption capacity after 13 cycles (see FIG. 4, which is a graphshowing the uptake and release of O₂ in Fe₂(dobdc) over 13 cycles at 211K. Adsorption occurred within 2 min upon application of 0.21 bar of O₂and desorption was carried out by placing the sample under dynamicvacuum for 25 min).

To predict how Fe₂(dobdc) would perform as an O₂/N₂ separation material,ideal adsorbed solution theory (ILAST) was employed at temperatures forwhich O₂ adsorption is reversible. The O₂ and N₂ isotherms measured at201, 211, 215, and 226 K were modeled with dual-site Langmuir-Freundlichfits. FIG. 15 is dual-site Langmuir-Freundlich fits for Oxygen (upper)and Nitrogen (lower) adsorption in Fe₂(dobdc). The continuous solidlines are the dual-site Langmuir-Freundlich fits using the parametersspecified in FIG. 45 (O₂) and FIG. 46 (N₂). FIG. 45 shows dual-siteLangmuir-Freundlich parameters for O2 isotherms in Fe2(dobdc) atdifferent temperatures. FIG. 46 shows dual-site Langmuir-Freundlichparameters for N2 isotherms in Fe2(dobdc) at different temperatures.

FIG. 16 is isosteric heats of adsorption of O₂ and N₂, determined fromthe dual-Langmuir-Freundlich fits in FIG. 45, FIG. 46 and application ofequation (2). Isosteric heats of adsorption calculated from these fitsare plotted in FIG. 16 and indicate higher enthalpies for O₂ adsorptionthan N₂ adsorption over the entire pressure range measured. The higherpropensity of O₂ to accept charge from Fe^(II) results in a largerinitial isosteric heat of −41 kJ/mol, as compared to that of N₂ (−35kJ/mol). Accordingly, Fe₂(dobdc) displays a high O₂/N₂ selectivity at201, 211, 215, and 226 K. FIG. 17 shows O₂/N₂ selectivity as a functionof pressure at 201 K, 211 K, 214 K, and 226 K. The selectivity rangesfrom 4.4 to over 11 and reaches a maximum of 11.4 at 201 K and about 0.4bar.

The high O₂/N₂ selectivity, in conjunction with the rapid and reversiblecycling times, suggest that Fe₂(dobdc) warrants further consideration asan adsorbent for O₂/N₂ separations via a modified vacuum-swingadsorption (VSA) process. FIG. 18 is a schematic drawing of VSAapparatus. Here, dry air is flowed over a packed bed of Fe₂(dobdc) attemperatures near 210 K, which could potentially offer significant costand energy savings over current separation technologies that areperformed at much lower temperatures. Breakthrough experiments weresimulated at 211 and 226 K to evaluate the performance of Fe₂(dobdc) forthe separation of O₂ from N₂ at concentrations similar to those presentin air (see FIG. 5). FIG. 5 is a graph that shows calculated N₂(diamonds) and O₂ (squares) breakthrough curves during adsorption ofsimulated air (O₂:N₂=0.21:0.79) by Fe₂(dobdc) at 211 K. The methodologyadopted for the breakthrough simulations has been described.²⁰ Thex-axis in FIG. 5 is a dimensionless time, τ, obtained by dividing theactual time, t, by the contact time between the gas and metal-organicframework crystallites, alu. For a given adsorbent, under selectedoperating conditions, the breakthrough characteristics are uniquelydefined by r, allowing the results presented here to be equallyapplicable to laboratory scale equipment as well as to industrial scaleadsorbers. It is apparent from the simulated curves that N₂ quicklysaturates the sample, as evidenced by the low breakthrough time. Themole % of N₂ in the outlet stream as a function of dimensionless time ispresented in FIG. 19, which is a calculated N₂ breakthrough curve duringadsorption of simulated air (O₂:N₂=0.21:0.79) by Fe₂(dobdc) at 211 K(lower left) and 226 K (upper left). Calculated O₂ breakthrough curveduring the desorption step of the vacuum-swing adsorption process at 211K (lower right) and 226 K (upper right). In contrast to currentlyemployed VSA processes, in which N₂ is selectively adsorbed on thepacked bed while O₂ is collected, Fe₂(dobdc) would selectively adsorbO₂.²¹ Accordingly, shortly after N₂ breakthrough, the gas stream is purenitrogen while O₂ is retained by the framework. Upon O₂ breakthrough,the VSA process is advanced to the second step, in which vacuum isapplied to the sample bed. Although the gas at the outlet is initially amixture of N₂ and O₂ the concentration of O₂ quickly increases to near100 mole % (see FIG. 19). This results in a large supply of pure O₂.After a majority of the O₂ is removed from the adsorber, a low-pressureflow of pure N₂ would be flowed over the material to fully regeneratethe bed for subsequent cycling.

Mössbauer Spectra. The different O₂ adsorption behavior at low versusroom temperature suggests the existence of two different modes by whichO₂ binds to the open iron sites in Fe₂(dobdc). Mössbauer spectroscopywas employed to probe the electronic structure at the metal center. Atall temperatures, the spectra of Fe₂(dobdc) in the absence of O₂ featurea simple doublet. At 298 K this doublet exhibits an isomer shift of1.094(3) mm/s and a quadrupole splitting of 2.02(1) mm/s. These valuesare consistent with high-spin iron(II) in a square pyramidalcoordination environment, as established below for the structure of thecompound. Upon exposure to O₂, a small amount (ca. 5-15%, depending upontemperature) of high-spin iron(II) is still observed, presumably becausea small portion of the iron(II) sites remain unoxygenated.

FIG. 6 is a Mössbauer spectra measured between 94 and 252 K forFe₂(dobdc) in the presence of O₂. As shown in FIG. 6, the spectrumobtained in the presence of O₂ at 94 K indicates that almost all of theiron in the sample has a substantially reduced isomer shift that isapproximately halfway between those expected for high-spin iron(II) andhigh-spin iron(III). This suggests a partial transfer of electrondensity from each of the Fe^(II) centers in the framework to form a weakbond with an O₂ species that is somewhere between the neutral moleculeand superoxide. Thus, exposure of Fe₂(dobdc) to O₂ at low temperaturesis consistent with the formation of Fe₂(O₂)₂(dobdc), featuring oneweakly held O₂ molecule per iron atom. This is fully consistent with theobservation of a reversible adsorption of 18.2 wt % O₂ at 211 K. At thispoint it is not possible to determine from the Mössbauer spectralresults whether the electron transfer is static or dynamic, with anelectron transfer time that is faster than the ca. 10⁻⁷ s time scale ofthe iron-57 Mössbauer spectral experiment.

Upon warming to 222 K and above, further changes arise in the Mössbauerspectra, which are clearly indicative of the formation of high-spiniron(III). The temperature at which this change in oxidation stateoccurs is consistent with the temperature at which we first observe theonset of and irreversible uptake of O₂ uptake in gas adsorptionexperiments (ca. 220 K). The change in oxidation state together with theirreversible uptake of 9 wt % O₂ suggest the formation of a compound offormula Fe₂(O₂)(dobdc), in which half of the Fe^(III) centers stronglybind a peroxide anion. Note that, consistent with the presence of atleast two different coordination environments, one with O₂ ²⁻ bound andone without, fitting the spectra requires the use of at least twodoublets for the iron(III) components.

The temperature dependence of the quadrupole splitting of main spectralcomponents observed for the framework in the presence of O₂,corresponding to the Fe^(II) centers in Fe₂(dobdc), the Fe^(II/III)centers in Fe₂(O₂)₂(dobdc), and the Fe^(III) centers in Fe₂(O₂)(dobdc),is shown in FIG. 20. As expected and in agreement with the Ingallsmodel,²² the quadrupole splitting of the square pyramidal high-spinFe^(II) center in Fe₂(dobdc) decreases the most with increasingtemperature, a decrease that results from changes in the electronicpopulation of the 3d_(xy), 3_(xz), and M_(yz) orbitals, whose degeneracyhas been removed by the low-symmetry component of the crystal field.Furthermore, there is a smaller decrease in the splitting upon warmingof the other two components. FIG. 21 is a temperature dependence of thelogarithm of the Mössbauer spectral absorption area of Fe₂(dobdc). Thetemperature dependence of the logarithm of the Mössbauer spectralabsorption area of Fe₂(dobdc) (see FIG. 21), is well fit with the Debyemodel for a solid²³ and yields a Debye temperature, Θ_(D), of 225(7) K,a value that is reasonable for the compound. Overall, the Mössbauer datapoint to a situation where, as a sample of Fe₂(dobdc) is warmed underO₂, an activation barrier is overcome for the transfer of electrons fromtwo different iron centers to form a bound peroxide anion at every otheriron site.

Infrared Spectra. The presence of various Fe—O₂ adducts as a function oftemperature should also be apparent by infrared spectroscopy. FIG. 7shows an infrared spectra obtained for Fe₂(dobdc) in the absence of O₂at room temperature, and upon dosing with 30 mbar of O₂ at roomtemperature, and at a low temperature near 100 K. Difference spectrabetween the bare and O₂ dosed materials at low and room temperature areshown. Spectra collected in transmission mode on thin films ofFe₂(dobdc) (see FIG. 7), reveal a number of framework vibrations below1300 cm⁻¹. The reactivity of Fe₂(dobdc) towards O₂ was followed at bothroom temperature and near 100 K. The series of spectra obtained at near100 K with varying O₂ loadings are shown in FIG. 24.

Part (a) of FIG. 24 shows IR spectra of activated Fe₂(dobdc); effect ofprogressive dosage of O₂ at low temperature. Maximum coverage 30 mbar.Part (b) of FIG. 24 shows the effect of outgassing at low temperature.It is evident that under these conditions the interaction with oxygen isfully reversible. Bands associated to the superoxo species are clearlyvisible at 1129, 541 and 511 cm⁻¹, while bands originally at 1250, 1198and at 580 cm⁻¹, shift to 1266, 1186 and 595 cm⁻¹ respectively.Animation of vibrational modes of Ni₂(dobdc) homologue on optimizedstructure computed with CRYSTAL code, reveal that all of them arerelated with the C—O bonds of the linker, being the bands at 1250 at 580cm⁻¹ strongly correlated. [1]

Oxygenation of Fe₂(dobdc) at low temperature gives rise to the spectrumindicated in FIG. 7, and the most relevant changes are evident in thedifference spectrum shown in magenta. New bands are seen at 1129, 541,and 511 cm⁻¹, while significant shifts are seen in the frameworks bandsoriginally at 1250, 1198, and 580 cm⁻¹ (causing negative components inthe difference spectrum). The component at 1129 cm⁻¹ is assigned toν(O—O) of a partially-reduced (near superoxo) O₂ species coordinated toFe^(II/III) sites. The first overtone for this stretching mode is alsoclearly visible at 2238 cm⁻¹. The band at 541 cm⁻¹ is associated withthe Fe—O₂ vibration of the of this species, whereas the band at 511 cm⁻¹is attributed to an Fe—O_(linker) mode of the framework, reflecting theO₂ adsorption induced modification in Fe.O_(linker) bonds.²⁴ Theinteraction with O₂ at low temperature is completely reversible byapplying vacuum to the sample cell.

Oxygenation of Fe₂(dobdc) at room temperature gives rise to the spectrumindicated in FIG. 7, which can be explained in terms of the formation ofa peroxo species coordinated to Fe^(III) centers.²⁵ The main features inthis case are a peak at 790 cm⁻¹, due to a ν(O—O) vibrational mode, anda pair of peaks at 697 and 670 cm⁻¹, arising from the peroxo ring modesof the Fe-(η²-O₂) unit. The peaks at 550 cm⁻¹ and 507 cm⁻¹ are furtherassigned to the ν_(asym) and ν_(sym) modes of the iron-oxygen bond ofthe peroxo species.

Similar features are more clearly visible in the ATR spectrum of anoxidized sample (see FIG. 25). FIG. 25 is an ATR spectra of activatedFe₂(dobdc) and of a partially oxidized sample. The spectra werecollected inside a N₂ filled glove box. Bands due to peroxo species areclearly visible at 796, 697 and 669 cm⁻¹, confirming the data obtainedin transmission mode on the sample carefully reacted at room temperature(see FIG. 7).

Small changes are also visible in the Raman spectrum of the sample uponO₂ interaction (see FIG. 26). FIG. 26 is a Raman spectra collected with512 nm laser at 5% of power on sample in a cell cooled by liquidnitrogen. Spectra of activated Fe₂(dobdc); partially oxidized sample;effect of 40 mbar of oxygen on activated sample. Raman spectra have beencollected on the sample cooled with a liquid nitrogen flux in order toreduce the laser damaging effects. In this case a band at 630 cm⁻¹ isthe most evident feature of the formation of a peroxo species. Smallerbands at 674 and 729 are also visible [2,3].

Overall, the vibrational spectra are fully consistent with the modelalready developed from interpretation of the O₂ adsorption data andMössbauer spectra.

Structures via Neutron Powder Diffraction. Powder neutron diffractiondata provide direct structural details of the means by which O₂ and N₂interact with Fe₂(dobdc) (see FIG. 8). FIG. 8 shows a first coordinationspheres for the iron centers within Fe₂(dobdc) and its O₂ and N₂ dosedvariants, as determined from Rietveld analysis of neutron powderdiffraction data. The structures depicted are for samples under vacuum(upper left), dosed with N₂ at 100 K (upper right), dosed with O₂ at 100K (lower left), and dosed with O₂ at 298 K (lower right). Alldiffraction data were collected below 10 K. Values in parentheses givethe estimated standard deviation in the final digit of the number.Initial data collected on an evacuated sample of Fe₂(dobdc) confirm thepresence of accessible Fe^(II) sites with a square pyramidalcoordination environment. Here, each iron center is coordinated by Odonor atoms from two aryloxide units (located at the front right andback left basal positions) and three carboxylate groups (at theremaining positions) from surrounding dobdc⁴⁻ ligands. Note that thearrangement of framework O donor atoms is the same in each depictionshown in FIG. 8.

FIG. 27 is Rietveld refinement of the experimental neutron diffractionpattern of Fe₂(dobdc) exposed to O₂ at 100K. The calculated pattern isin good agreement with the experimental data (crosses) as evidenced bythe difference pattern. A Rietveld refinement was performed against datacollected for a sample of Fe₂(dobdc) that was cooled to 100 K, dosedwith two equiv of O₂ per iron, and then cooled to 4 K (see FIG. 27).Three different O₂ adsorption sites are evident in the resulting model.The highest occupancy site, with a refined occupancy of 0.917(8) O₂molecules per iron, is located at the open iron coordination position.Significantly, the O₂ molecule binds in a symmetric side-on coordinationmode, with Fe—O distances of 2.09(2) and 2.10(1) Å. The O—O separationof 1.25(1) Å lies between the internuclear distances observed for freeO₂ (1.2071(1) Å)²⁶ and typical of an O₂ ⁻ superoxide unit (1.28 Å).⁴This again is consistent with only partial reduction of O₂ under theseconditions. Although symmetric side-on coordination of superoxide andperoxide to other transition metals has been reported²⁷ this represents,to the best of our knowledge, the first crystallographic evidence ofnon-bridging side-on binding of any dioxygen species to iron in anon-enzyme system.²⁸ The second and third O₂ adsorption sites, withoccupancies of 0.857(9) and 0.194(8), respectively, occur in the poresof the framework at distances of greater than 3 Å from the iron centerand organic linker (see FIG. 28), indicating weak dispersive typeinteractions between the adsorbate and the framework walls.

FIG. 28 is Rietveld refinement of the experimental neutron diffractionpattern of Fe₂(dobdc) exposed to O₂ at 298K. The calculated pattern isin good agreement with the experimental data (crosses) as evidenced bythe difference pattern.

FIG. 29 shows a structure of Fe₂(O₂)₂(dobdc)-2O₂ as viewed down the(001) direction. (H atoms have been omitted for clarity.) There is anethylene molecule bound to the open coordination site at each iron(II)center. At the bottom are an H₄(dobdc) ligand and the first coordinationspheres for the iron centers in the solid state structures obtained upondosing Fe₂(dobdc) with acetylene, ethylene, ethane, propylene, andpropane. Note that for propane in Fe₂(dobdc) the adsorbed hydrocarbonmolecule has orientational disorder with respect to the open metalcenter. Of several refined models, the single-molecule with largedisplacement parameters is the most reasonable.

Rietveld refinement performed against data collected on a sample ofFe₂(dobdc) that had been dosed with an excess of O₂ at room temperature,evacuated, and subsequently cooled to 4 K was also performed (see FIG.29). The data were best fit by a model in which O₂ is coordinated toiron in an asymmetric side-on mode and at a refined occupancy of0.46(2). The model indicates substantial elongation of the O—O distanceto 1.6(1) Å, consistent with a two-electron reduction of O₂ to peroxide.With an Fe—O₂ centroid distance of 2.26(1) Å, the peroxide unit alsoappears to have slipped substantially towards one of the bridgingligands. This type of coordination of peroxide has been observedpreviously in naphthalene dioxygenase^(28a) and has also been proposedbased upon spectroscopic evidence for a number of non-heme ironcomplexes.²⁹

Neutron powder diffraction data were further collected on a sample ofFe₂(dobdc) dosed with 0.5, 1.0, and 2.0 equiv of N₂ dosed at 80 K. Upondosing with approximately 0.5 equiv of N₂, a binding site at the metalcenter is apparent with an occupancy of 0.641(5). Nitrogen coordinatesend on with an Fe—N—N angle of 179(1)° and an Fe—N distance of 2.30(1)Å. The N—N distance of 1.133(15) Å is slightly longer than the N—Ndistance of free nitrogen (1.0977(1) Å).³⁰ Additional N₂ uptake revealsa second site that runs more parallel to the pore walls, with N . . . Ocontacts between 3.4 and 3.6 Å. The close N₂-framework interactions arethe origins of the relatively high enthalpy for adsorption. Themetal-specific interactions, however, are clearly much weaker than forO₂, which results in interaction of both atoms with the metal, electrontransfer, and a significant compression of the unit cell upon adsorption(see FIG. 48). FIG. 48 shows a table of unit cell lengths and volumes.

The differences in how O₂ binds to iron within Fe₂(dobdc) at low versushigh temperatures suggests that the framework undergoes electrontransfer processes similar to those reported for nonheme iron-containingenzymes.³¹ In these systems, O₂ typically progresses through a number ofelectron transfer steps starting with superoxo and peroxo. In the caseof Fe₂(dobdc) at low temperature, each iron shares one of its electronswith a single O₂ molecule, resulting in oxidation of all of the metalcenters to an intermediate iron(II/III) oxidation state. This chargetransfer is reversible at low temperatures and accounts for the high gasuptake demonstrated in the gas adsorption experiments. However, atelevated temperatures two electrons are transferred to the adsorbing O₂molecule, the first presumably being shared in a manner analogous towhat occurs at low temperature, and the second subsequently arrivingfrom an adjacent iron center by promotion over an activation barrier viathe available thermal energy. In this scenario, all of the metal centerswithin the framework are converted to iron(III), half of which arecoordinated irreversibly to a peroxide anion, while the other halfremain five-coordinate.

The foregoing results demonstrate the ability of Fe₂(dobdc) toselectively bind O₂ over N₂ via electron transfer interactions.Breakthrough curves calculated using single-component gas adsorptionisotherms and ideal adsorbed solution theory indicate that the materialshould be capable of the high-capacity separation of O₂ from air attemperatures as high as 226 K. This is substantially higher than thecryogenic temperatures currently used to separate O₂ from air on a largescale. At still greater temperatures, a thermal activation barrier tothe formation of iron(III)-peroxide species is overcome and desorptionof O₂ is no longer possible. Efforts are underway to synthesize relatedmetal-organic frameworks with an increased activation barrier for theformation of peroxide, thereby generating a high-capacity O₂ separationmaterial that can operate closer to ambient temperatures.

In addition, the efficacy of the new redox-active framework inperforming a variety of other gas separations where charge transfermight also lead to selectivity. Additional example separations include,but are not limited to, paraffin/olefin separations, carbon monoxideremoval, acetylene storage, and nitric oxide/nitrous oxide separations.

FIG. 9 is a graph showing the separation of a mixture of ethane andethane. A 50/50 mixture of ethane and ethene is flowed throughFe₂(dobdc) at 318 K. The framework adsorbs ethene first, supplyinggreater than 99.5% purity ethane. After ethene “breaks through” gas feedis turned off to supply greater than 99% purity ethene.

FIG. 10 is a graph showing the separation of a mixture of propane andpropene. A 50/50 Mixture of propane and propene is flowed throughFe₂(dobdc) at 318 K. The framework adsorbs propene first, supplyinggreater than 99.5% purity propane. After propene “breaks through” gasfeed is turned off to supply greater than 99% purity propene.

To investigate the ability of Fe₂(dobdc) to adsorb light hydrocarbons,pure component equilibrium adsorption isotherms for methane, ethane,ethylene, acetylene, propane and propylene were measured at 318, 333,and 353 K. FIG. 30 shows the data obtained at 318 K, with the remainingdate presented in FIG. 11. FIGS. 30 (a) and (b) show gas adsorptionisotherms for methane, ethane, ethylene, and acetylene (a) and forpropane and propylene (b) in Fe₂(dobdc) at 318 K. Filled and opencircles represent adsorption and desorption data, respectively. Theadsorption capacities at 1 bar correspond to 0.77, 5.00, 6.02, 6.89,5.67, and 6.66 mmol/g, respectively. FIGS. 26 (c) and (d) areexperimental breakthrough curves for the adsorption of equimolarethane/ethylene (c) and propane/propylene (d) mixtures flowing through a1.5 mL bed of Fe₂(dobdc) at 318 K with a total gas flow of 2 mL/minuteat atmospheric pressure. After breakthrough of the olefin and return toan equimolar mixture composition, a nitrogen purge was applied, leadingto desorption of the olefin. Note that in an actual separation scenario,desorption would instead be carried out by applying a vacuum and/orraising the temperature. As evidenced by the initial steep rise in theisotherms, Fe₂(dobdc) displays a strong affinity for the unsaturatedhydrocarbons acetylene, ethylene, and propylene. Additionally, theuptake of these gases at 1 bar approaches the stoichiometric quantityexpected if one gas molecule is adsorbed per iron (II) center. Thepropane and ethane adsorption capacities under these conditions, thoughlower than those of their unsaturated counterparts, are bothsignificantly higher than observed for methane, which has lowerpolarizability and a smaller kinetic diameter. Importantly, all of theisotherms are completely reversible and exhibit no hysteresis. Further,equilibrium adsorption experiments at 318K (FIG. 12) indicate no loss inolefin uptake capacity after 15 ethylene adsorption/desorption cycles.Additionally, no loss in propylene uptake was observed after 40adsorption/desorption cycles as verified by thermogravimetric analysis(FIG. 12).

Powder neutron diffraction experiments were carried out to determine thenature of the interactions of these adsorbate molecules withinFe₂(dobdc). In a typical experiment, Fe₂(dobdc) was dosed withdeuterated gas at 100 K and cooled to 4 K for data collection. Rietveldrefinements were performed agains these data to provide the structuralmodels presented in FIG. 28. Analogous to the results obtainedinvestigating the coordination of dioxygen to the iron centers of thismaterial, only one adsorption site is apparent. This site corresponds tothe open coordination site of the exposed Fe²⁺ cations, upon dosingsub-stoichiometric equivalents of gas per framework iron. Theunsaturated hydrocarbons, acetylene, ethylene, and propylene, displaythe anticipated side-on binding modes, with Fe—C distances lying in therange 2.42(2) to 2.60(2) Å. These distances are substantially longerthan the separations of 2.020(5) to 2.60(2) Å observed for thediamagnetic complex [Fe(C₂H₄)₄]², one of the very few iron(II)-olefinspecies to be structurally characterized previously. The differencesuggests that the metal centers within Fe₂(dobdc) maintain a high-spinelectron configuration when binding these gases, consistent with weakerinteractions that can be reversed with little energy penalty. Theinteractions of both ethane and propane with the metal cations inFe₂(dobdc) are weaker, as evidenced by the elongated Fe—C distance ofapproximately 3 Å. This is in good agreement with the Mg—C distancereported for methane adsorption in Mg₂(dobdc), a system in which themetal-adsorbate interactions are also a result of ion-induced dipoleinteractions between coordinatively-unsaturated metal cations andhydrocarbon deuterium atoms.

The strength of the hydrocarbon binding with Fe₂(dobdc) was determinedquantitatively through analysis of the gas adsorption data. The data foracetylene, ethylene, ethane, propane, and propylene, expressed in termsof absolute loadings, were fitted with the dual-Langmuir-Freundlichisotherm model, whereas methane adsorption data were fitted with asingle-site Langmuir model. Isosteric heats of adsorption werecalculated form the fits to compare the binding enthalpies of thesegases under various loadings (see FIG. 13). Heats of adsorption foracetylene (−47 kJ/mol), ethylene (−45 kJ/mol), and propylene (−44kJ/mol) show a significant reduction as the loading approaches the valuecorresponding to one gas molecule per iron(II) center, again consistentwith the exposed metal cations presenting the strongest adsorption sitesin the material. Propane (−33 kJ/mol), ethane (−25 kJ/mol), and methane(−20 kJ/mol) adsorption enthalpies are all considerably lower inmagnitude, with the trend reflecting the decreasing polarizabilities ofthese molecules from propane to ethane to methane.

Adsorption selectivities were calculated using ideal adsorbed solutiontheory (IAST) using the fitted isotherms of the experimental isothermdata for relevant gas mixtures in Fe₂(dobdc) and a number of otherporous material for which analogous gas uptake properties have beenreported (see FIG. 31). FIG. 31 shows calculations of the adsorptionselectivity, S_(ads), using Ideal Adsorbed Solution Theory forethane/ethylene (upper left), propane/propylene (upper right),acetylene/ethylene (lower left) and acetylene/methane, ethylene/methane,ethane/methane (lower right) in Fe₂(dobdc) at 318 K. For the equimolarmixture of ethylene and ethane at 318 K, the adsorption selectivitiesobtained for Fe₂(dobdc) are significantly greater than those calculatedfor either zeolite NaX or the isostructural metal-organic frameworkMg₂(dobdc), which display seletvities of 9-14 and 4-7, respectively. Thelatter result is consistent with the softer character of Fe²⁺ relativeto Mg²⁺, leading to a stronger interaction with the π electron cloud ofthe olefin. Similarly, in comparing the performance of Fe₂(dobdc) withother porous materials for the separation of a propane/propylene mixture(selectivity=13-15), it is rivaled in selectivity only by zeolite ITQ-12which displays adsorption selectivity of 15 while the other materialsdisplay selectivities from 3-9. Note, however, that the selectivities ofITQ-12 for this mixture were calculated from data collected at 303 K,and it is expected that selectivity of this material will be lower athigher temperatures. Adsorption selectivities were also calculated usingLAST for Fe₂(dobdc) in an equimolar four-component mixture of methane,ethane, ethylene, and acetylene at 318 K, as relating to thepurification of natural gas. For an adsorption-based process operatingat 1 bar, the calculated acetylene/methane, ethylene/methane, andethane/methane selectivities are 700, 300, and 20, respectively. Thesevalues are much higher than those recently reported (13.8, 11.1, and16.6, respectively) for a zinc-based metal-organic framework, also basedon an analogous calculation procedure.

To evaluate performance of Fe₂(dobdc) in an actual adsorption-basedseparation process, breakthrough experiments were performed in which anequimolar ethylene/ethane or propylene/propane mixture was flowed over apacked bed of the solid with a total flow of 2 mL per minute at 318 K(see FIG. 32 and FIG. 33). FIG. 32 is curves showing mol % (left) andconcentration (right) of propane and propylene during adsorption (upper)and desorption (lower) of a simulated breakthrough experiment. FIG. 29is curves showing mol % (left) and concentration (right) of ethane andethylene during adsorption (upper) and desorption (lower) of a simulatedbreakthrough experiment. In a typical experiment, the gas mixture wasflowed through 300 to 400 mg of metal-organic framework crystallitespacked into 1.5 mL glass column, and the outlet gas stream was monitoredby a gas chromatograph equipped with a flame ionization detector. Asexpected from the calculated selectivites, in each case, the alkane wasfirst to elute through the bed, while the solid adsorbent retained theolefin. For the C₃ hydrocarbons, the outlet gas contained undetectablelevels of propylene, resulting in a propane feed that appeared to be100% pure, within the detection limit of the instrument (˜100 ppm). Uponsaturation of the metal centers within the adsorbent, propylene “brokethrough” and the outlet gas stream then quickly reached equimolarconcentrations. By stopping the gas feed and flowing a purge of nitrogenthrough the bed, the small amount of weakly bound propane remaining inthe pores of the framework could be quickly removed, while theiron-bound propylene then desorbed more slowly. Greater than 99% purepropylene was realized during the desorption step of the breakthroughexperiment. In a similar manner, breakthrough experiments showed thatFe₂(dobdc) can separate an equimolar mixture of ethylene and ethane intothe pure component gases of 99% and 99.5% purity.

Although breakthrough experiments are quite valuable for evaluating thegas separation capabilities of a material, in practice they can bedifficult and time consuming. In order to compare Fe₂(dobdc) with otherreported adsorbents for ethylene/ethane and propylene/propaneseparations, we sought to demonstrate that the breakthroughcharacteristics could instead be simulated with reasonable accuracy.Assuming that (i) intra-crystalline diffusion is negligible through anisothermal adsorption bed in thermodynamic equilibrium; (ii) plug flowproceeds through the bed; and (iii) the binary mixture adsorptionequilibrium in the packed bed of crystallites can be calculated usingLAST, we were able to solve a set of partial differential equiations andcalculate breakthrough curves for both ethylene/ethane andpropylene/propane mixtures. The resulting transient gas compositionprofiles (see FIG. 16 and FIG. 17) are in excellent agreement with theexperimental results shown in FIG. 32 and FIG. 33.

Given this validation, analogous simulations were employed to makequantitative comparisons with other materials. From the simulatedbreakthrough curves, the time interval during which the exit gascompositions have a purity of 99% propane can be determined, togetherwith the amount of 99% pure propane produced in this time interval. Theproduction capacities, expressed as the amount of propane produced perliter of adsorbent are shown in FIG. 34 over a range of pressures forthe zeolites ITQ-12 at 303 K and NaX at 318 K, and for the metal-organicframeworks Cu₃(btc)₂ (btc³⁻=1,3,5-benzenetricarboxylate) at 318 K,Cr₃(btc)₂ at 298 K, and Fe-MIL-100 at 303 K. FIG. 34 (left) isproduction capacity of 99% pure propane, expressed as mol propaneproduced per L adsorbent material, as a function of the total pressureat the inlet to the adsorber. The separation characteristics ofFe₂(dobdc) at 318 K are compared to that of Mg₂(dobdc) (318 K), NaXzeolite (318 K), Cu₃(btc)₂ (318 K), Cr₃(btc)₂ (298 K), ITQ-12 (303 K),and Fe-MIL-100 (303 K). (Right) is the production capacity of 99.5% purepropylene, expressed as mol propane produced per L adsorbent material,as a function of the total pressure at the inlet to the adsorber. Theseresults indicate that the propane production capacity of Fe₂(dobdc) at318 K, which ranges up to 5.8 mol/L at a total pressure of 1.0 bar, isat least 20% higher than that of any of these other materials. A similarmethod was used to calculate the amount of polymer-grade (99.5%+)propylene that can be produced by these materials, again leading to ahigher capacity for Fe₂(dobdc) than for any other material. The compoundMg₂(dobdc) exhibits a lower productivity than Fe₂(dobdc), a result ofthe lower adsorption selectivity of this material. Although zeoliteITQ-12 displayed a comparable selectivity to Fe₂(dobdc), its capacitylimitation, which stems from its low pore volume of 0.134 cm³/g, resultsin a propylene productivity that is just 47% of that of themetal-organic framework.

For the separation of ethylene/ethane mixtures, the breakthroughsimulations indicate an even greater advantage of Fe₂(dobdc) over otheradsorbents, with production capacities that are roughly double those ofMg₂(dobdc) and zeolite NaX (see FIG. 35). FIG. 35 is productioncapacities of 99% pure ethane (left), and 99.5% pure ethylene (right),expressed as mol produced per L adsorbent material, as a function of thetotal pressure at the inlet to the adsorber. The separationcharacteristics of Fe₂(dobdc) at 318 K are compared to that ofMg₂(dobdc) (318 K), and NaX zeolite at temperatures of 298 K, and 323 K.

In order to establish the feasibility of using Fe₂(dobdc) for the taskof selectively separating methane from mixtures including C₂hydrocarbons (ethane, ethylene, and acetylene), breakthroughcalculations were carried out for the mixture. The graph on the left ofFIG. 36 shows calculated methane, ethane, ethylene, and acetylenebreakthrough curves for an equimolar mixture of the gases at 1 barflowing through a fixed bed of Fe₂(dobdc) at 318 K. FIG. 36 presentssimulated data on the gas phase molar concentrations exiting an adsorberpacked with Fe₂(dobdc) and subjected to a feed gas consisting of anequimolar mixture of methane, ethane, ethylene, and acetylene at a totalpressure of 1 bar and a temperature of 318 K. Note that the breakthroughtimes reflect the relative adsorption selectivities(acetylene>ethylene>ethane>methane) for the material, and that thecurves indicate a clean, sharp breakthrough transition for eachsuccessive gas.

Based on these results, the diagram at the right in FIG. 36 demonstrateshow it might be possible to procure pure methane, ethane, ethylene, andacetylene using three packed beds of Fe₂(dobdc). The diagram on theright in FIG. 36 shows a schematic representation of the separation of amixture of methane, ethane, ethylene, and acetylene using just threepacked beds of Fe₂(dobdc) in a vacuum swing adsorption or temperatureswing adsorption process. In this process, a gas mixture is fed into thefirst bed and methane, the fraction with the lowest adsorptivity, breaksthrough first. Pure methane can be collected until the second gas,ethane, breaks through. When the third component of the gas stream,ethylene is present in the eluent, the gas flow is diverted to a secondbed, from which additional pure methane is collected during theadsorption step, and from which a mixture of ethane and ethylene issubsequently desorbed. This ethane/ethylene mixture is then separatedinto its pure components using a third adsorbent bed. By halting thefeed into the first bed just prior to breakthrough of acetylene, pureacetylene can be obtained via desorption.

The use of Fe₂(dobdc) for removal of acetylene from mixtures withethylene was investigated. FIG. 37 is transient breakthrough ofacetylene/ethylene mixture in an adsorber bed packed with Fe₂(dobdc).The inlet gas is maintained at partial presses p₁=100 kPa, p₂=1 kPa, ata temperature of 318 K. Simulated breakthrough characteristics for afeed mixture containing 1 bar of ethylene and 0.01 bar of acetylene at318 K indicate that final acetylene concentrations on the order of 10ppm could be realized (see FIG. 37).

Fe₂(dobdc) can also be used for carbon monoxide separation. Fe2(dobdc)for the selective adsorption of carbon monoxide from H₂, N₂, and CH₄ wasinvestigated. FIG. 41 shows the Excess adsorption isotherms of CO, CH₄,N₂, and H₂ collected for Fe₂(dobdc) at 318 K. Isosteric heat of COadsorption in Fe₂(dobdc) as a function of loading. (inset).Single-component equilibrium adsorption isotherms collected on a sampleof Fe₂(dobdc) at 318 K (FIG. 41) indicate significantly higher uptake ofCO than N₂, H₂, or CH₄ over the entire pressure range measured. The COisotherm displays a steep rise at low pressure and saturates over 12 wt% at 1 bar, while the other gases measured remain below 1.3 wt % at thispressure. Accordingly, the isosteric heat of CO adsorption (41 kJ/mol)as calculated from fits to isotherms collected at multiple temperaturesis significantly higher than N₂, H₂, and CH₄ (35, 10.1, 20 kJ/mol,respectively). The high adsorption enthalpy of carbon monoxide to theFe²⁺ centers in Fe₂(dobdc) as compared to N₂, CH₄, and H₂ indicate thatthis material may be used for the adsorptive separation of these gases.

FIG. 38 is a portion of the structure of Fe₂(dobdc) showing carbonmonoxide (CO) coordinated to the open Fe²⁺ site. FIG. 39 is an infraredspectrum of Fe₂(dobdc) under successively increasing doses of CO. FIG.40 is low-pressure adsorption of CO, CH₄, N₂, and H₂ in Fe₂(dobdc) at298 K.

Infrared spectroscopy was used to investigate the nature of theinteraction between the iron cations in Fe₂(dobdc) and adsorbed carbonmonoxide. FIG. 42 show the Infrared spectrum of CO adsorbed onFe₂(dobdc) at room temperature. Although infrared spectra formetal-organic frameworks of this structure type have a large number ofvibrations at lower frequency, the pertinent region for metal carbonylstretches is unobscured. Indeed, upon dosing an activated frameworksample with CO at room temperature a clear band is evident at 2160 cm⁻¹.With higher CO loading this band intensifies and shifts slightly to 2159cm⁻¹. At high coverage a number of additional bands appear at 2110,2125, and 2190 cm⁻¹. The CO band at 2159 cm⁻¹ is assigned to theiron-bound carbonyl species, near the shift that has been previouslyreported for CO bound to the metal cation centers in Mg₂(dobdc) andNi₂(dobdc). While the IR bands seen for these three materials are allblue-shifted relative to the IR stretch of gas phase CO (2143 cm⁻¹), theshift seen for Fe₂(dobdc) is red-shifted considerably compared to the Mgand Ni materials. This is likely attributed to the back-bonding from theFe²⁺ to the 2π* antibonding orbital of CO weakening the C—O bond ascompared to CO bound to Mg₂(dobdc) and Ni₂(dobdc).

Powder neutron diffraction experiments were also carried out tocrystallographically characterize (Fe—CO)₂(dobdc). Desolvated Fe₂(dobdc)was dosed with 0.75, and subsequently 1.5, equivalents of CO (per Fe²⁺)at 300 K and cooled to 4 K for data collection. FIG. 43 show Crystalstructures of Fe₂(dobdc) with adsorbed N₂, CO, or D₂. Carbon, iron,oxygen, nitrogen, and deuterium atoms are shown. At sub-stoichiometic COloadings only one adsorption site is apparent, with an occupancy of0.84(1). Consistent with infrared spectroscopy and gas adsorptionexperiments the strongest binding site is the unsaturated iron cationcenter. Carbon monoxide binds to the iron center end on at a Fe—Cdistance of 2.22(2) Å. The C—O distance for this complex of 1.11(2) isslightly shorter than that of free CO (1.127) consistent with thecorresponding blue shift seen via IR. CO forms a nearly linear adductwith a Fe—C—O angle of 172.6(1)°. At the higher CO loading of 1.5 perFe²⁺ the first binding site saturates at an occupancy of 0.96(2) and asecond site is apparent in which CO is parallel to the pore wall at adistance of 3.3-3.6 Å and an occupancy of 0.44(2).

Finally, given its clear ability to activate O₂, Fe₂(dobdc) can beemployed as a catalyst for the oxidation of hydrocarbons. For example,Fe₂(dobdc) reacts rapidly in air to produce either Fe₂(O₂)₂(dobdc) (lowtemperature) or Fe₂(O₂)(dobdc) (room temperature) both of which containreactive oxygen, either as superoxide in the former or peroxide in thelatter. The large pore volume, high surface areas, accessible metalcenters, and thermally stable nature of both of these resultingmaterials make them very promising oxidation catalysts. Although workwith a number of systems, including the oxidation of methane to methanoland the oxidation of ethane/ethane and propane/propene. A representativereaction is shown in FIG. 49.

Fe₂(dobdc) catalyzes the oxidation of propylene to acetone with air asthe oxidant. Although the yield of the reaction under current conditionsis low the selectivity is approximately 100%. Examples of otherreactions that may be chaptalized are shown below in FIG. 50.

Liquid-phase Hydrocarbon Oxidation Using Fe₂ (dobdc). In molecularFe-oxo chemistry, N-oxides, peroxides, and hypervalent iodine-basedO-atom transfer reagents are often used as sacrificial oxidants togenerate the reactive Fe species from initial Fe^(II) complexes.Fe₂(dobdc) was examined for liquid-phase C—H activation studiesinvolving O-atom transfer reagents. While only limited reactivity wasobserved using pyridine-N-oxide and iodosylbenzene, upon addition of asolution of 2-(tert-butylsulfonyl)iodosylbenzene (tBuSO₂PhIO) (5 equiv.)and excess 1,4-cyclohexadiene (1,4-CHD) (24 equiv.) in CD₃CN to theacetonitrile-solvated framework production of benzene (70% conversionbased on iodosylarene) was observed (FIG. 1). A control experiment inwhich no metal-organic framework was added led to less than 3% benzeneconversion. In addition, preliminary reactivity studies show that theframework is also capable of facilitating the room temperatureconversion of toluene to benzyl alcohol/benzaldehyde and cyclohexane tocyclohexanol/cyclohexanone, albeit in low yields (<10% and <3%,respectively) with modest alcohol:ketone selectivities (3.5:1 and 4:1,respectively). Despite low yields and selectivities, these preliminaryexperiments provide indirect evidence that the oxidized iron frameworkis a strong oxidant, capable of functionalizing even the C—H bonds ofcyclohexane (BDE=99.3 kcal/mol).

Gas-phase Oxidation of Fe₂(dobdc). Using a fully desolvated frameworkand a gaseous oxidizing agent without C—H bonds would circumvent anypossible side-reactions such as H-atom abstraction. For this reason, weexamined the gas phase oxidation of Fe₂(dobdc) with oxidants such as N₂Oor O₂. The reactivity of Fe₂(dobdc) sample oxidized with a gas phaseoxidant was probed using 1,4-CHD. After addition of neat, excess1,4-CHD, the framework gradually changed color from dark red brown tolight yellow (similar in color to the methanol-solvated framework).Benzene was formed in 40% yield, demonstrating that the framework iscapable of C—H activation.

Catalytic Oxidation of 1,4-Cyclohexadiene by Fe₂(dobdc). Fe₂(dobdc) wasadded to a stirring solution of excess 1,4-cyclohexadiene and2-(tert-butylsulfonyl)iodosylbenzene in CD₃CN.1,2,4,5-tetramethylbenzene was added as an internal standard. Thereaction mixture was stirred overnight at room temperature, filtered,and washed with 1 mL of CD₃CN. Reduction of the iodosylarene to2-(tert-butylsulfonyl)iodobenzene (quantitative) and formation ofbenzene (70% yield, assuming 1 equiv of iodosylarene consumed leads to 1equiv of benzene produced) was detected by ¹H NMR. A ¹H NMR of1,4-cyclohexadiene was taken to quantify the amount of benzene in thestarting material (1.3%). An identical control reaction run withoutFe2(dobdc) was also performed, leading to ˜3% conversion by ¹H NMR. FIG.44 shows the ¹H NMR of products of the reaction of Fe₂(dobdc) withexcess 1,4-cyclohexadiene in CD₃CN.

This application discloses several numerical ranges in the text andfigures. The numerical ranges disclosed inherently support any range orvalue within the disclosed numerical ranges even though a precise rangelimitation is not stated verbatim in the specification because thisinvention can be practiced throughout the disclosed numerical ranges.

The above description is presented to enable a person skilled in the artto make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein. Finally,the entire disclosure of the patents and publications referred in thisapplication are hereby incorporated herein by reference.

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What is claimed:
 1. A method of separating a mixture stream comprising aparaffin and an olefin comprising: contacting the mixture stream withFe₂(dobdc)(dobdc=2,5-dioxido-1,4-benzenedicarboxylate); and obtaining afirst stream richer in the paraffin as compared to the mixture stream.2. The method of claim 1, wherein the paraffin is ethane and the olefinis ethene.
 3. The method of claim 1, wherein the paraffin is propane andthe olefin is propene.
 4. The method of claim 1, wherein the paraffin inthe obtained first stream has a purity greater than 99.5 percent.
 5. Themethod of claim 1, further comprising obtaining a second stream richerin the olefin as compared to the mixture stream.
 6. The method of claim1, wherein the temperature of the mixture stream when contacting withFe₂(dobdc)(dobdc=2,5-dioxido-1,4-benzenedicarboxylate) is 318 to 353 K.7. The method of claim 6, wherein the pressure of the mixture streamwhen contacting withFe₂(dobdc)(dobdc=2,5-dioxido-1,4-benzenedicarboxylate) is equal to orless than 1 bar.
 8. The method of claim 1, wherein the temperature ofthe mixture stream when contacting withFe₂(dobdc)(dobdc=2,5-dioxido-1,4-benzenedicarboxylate) is 201 to 226 K.9. The method of claim 5, wherein the olefin in the second stream has apurity greater than 99 percent.